观察者模式(jdk实现)

1.定义

在对象中定义一对多的依赖,当一个对象改变状态,依赖它的对象会收到通知并更新.

2.实现 (主要通过jdk自己定义的观察者实现)

以气象站通知展示板为例子,当气象站收到的各种参数改变的时候,就通知展示板并显示收到的参数.

定义一个气象站,继承jdk的Observable类

public class WeatherData extends Observable{
    private float temperature;
    private float humidity;
    private float pressure;
    public WeatherData() {
    }
    public void measurementChanged() {
        setChanged();
        notifyObservers();
    }
    public void setMeasurements(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        measurementChanged();//改变状态并通知
    }
    public float getTemperature() {
        return temperature;
    }
    public float getHumidity() {
        return humidity;
    }
    public float getPressure() {
        return pressure;
    }
}

定义观察者———展示板,并且实现了jdk的Observer接口,代码如下

public class CurrentConditionsDisplay implements Observer, DisplayElement{
    Observable observable;
    private float temperature;
    private float humidity;
    public CurrentConditionsDisplay(Observable observable) {
        this.observable = observable;
        observable.addObserver(this);
    }
    @Override
    public void update(Observable o, Object arg) {
        if (o instanceof WeatherData) {
            WeatherData weatherData = (WeatherData)o;
            this.temperature = weatherData.getTemperature();
            this.humidity = weatherData.getHumidity();
            display();
        }
    }
    @Override
    public void display() {
        System.out.println(toString());
    }
    @Override
    public String toString() {
        return "CurrentConditionsDisplay [temperature=" + temperature + ", humidity=" + humidity + "]";
    }
}

测试类如下

public class ObserverTest {
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        CurrentConditionsDisplay display = new CurrentConditionsDisplay(weatherData);
        weatherData.setMeasurements(1.2F, 1.2F, 1.3F);
    }
}